This invention relates to a method of adjusting thermal dependence of an optical fiber. In addition, the invention relates to the adjustment of thermal dependence in a Mach-Zehnder interferometer (MZI) having two different fibers as its arms.
Optical fibers are used in various applications, such as Mach-Zehnder interferometers, signal combiners, optical filters and the like. For most of these applications, a control of the thermal dependence is required.
For example, an adjustment of the thermal dependence may be required when the fibers used to form the two arms of the MZI have different dopants to achieve the required optical path-length difference. Such dissimilar fibers usually present a different thermal dependence of the effective index of refraction in each of the two arms, which difference should be reduced as much as possible to achieve proper operation of the MZI.
It is an object of the present invention to provide a simple and effective method for adjustment of the thermal dependence of an optical fiber.
Another object is to provide adjustment of the thermal dependence of an all-fiber Mach-Zehnder interferometer having two different fibers as its arms.
A further object is to effect the adjustment of the thermal dependence in the MZI essentially without affecting the optical phase shift or spectral response of the interferometer.
Other objects and advantages will be apparent from the following description of the invention.
The present invention comprises subjecting an optical fiber, such as one of the optical fibers of a Mach-Zehnder interferometer having two different fibers as arms, to hydrogenation to make it photosensitive, followed by exposure of the photosensitive fiber to ultraviolet radiation (UV) for a predetermined period of time so as to adjust the thermal dependence of the fiber to a desired degree. This results in a decrease of the thermal dependence of the effective index of the fiber. In essence, therefore, the present invention provides a method of adjusting the thermal dependence of a photosensitive optical fiber by exposing it to UV radiation for a predetermined period of time. In the MZI, this can be achieved without affecting the optical phase shift between the arms of the MZI. Further, the decrease is greater as the time of exposure to the UV is greater, thus permitting a desired adjustment of the thermal dependence.
It is already known from U.S. Pat. No. 5,235,659 to produce large normalized refractive index changes in optical waveguides by hydrogenation of the fiber at a temperature of at most 250xc2x0 C. and then irradiating the so exposed fiber with UV radiation.
U.S. Pat. No. 5,478,371 discloses an improvement of the above method by first subjecting the fiber to hydrogenation, followed by heating and simultaneous UV radiation. This is stated to produce a substantive and long lived increase in the refractive index of the irradiated region.
U.S. Pat. No. 5,930,420 discloses a further improvement which provides for heating the hydrogenated glass to a temperature in excess of 500xc2x0 C., then permitting the glass to rapidly cool below 100xc2x0 C., followed by exposure to UV radiation. This is stated to increase the index of refraction of the so treated region.
Moreover, from U.S. Pat. No. 6,075,596 it is already known to provide one of the arms of a Mach-Zehnder interferometer with a length of photosensitive fiber and then exposing it to deep UV light. This is done for adjusting the optical path-length so as to enhance the bandwidth of the fiber optic circulator which is the object of that patent.
Finally, U.S. Pat. No. 6,221,566 provides for a method of enhancing the photosensitivity of an optical waveguide by loading it with H2 or D2 to form an associated base line refractive index, then exposing it to UV radiation to induce a change in the base line refractive index and finally annealing the waveguide to diffuse the gas from the loaded optical waveguide and to stabilize the change in the refractive index.
All this prior art deals with adjustment or control of the refractive index in an optical fiber or waveguide, and none of it provides any indication that hydrogenation of an optical fiber to render it photosensitive, followed by exposure to UV could control the thermal dependence of the fiber, or of a device, such as an MZI in which the fiber is used, without essentially affecting the optical path-length difference between the arms of the MZI.